The recommendation so far is for a medium speed fan with speed adjustment and isolated from the tube to provide minimum vibration blowing accross the surface of the mirror and a temperature probe for the mirror and ambent. As a start for me will be simply a fan on top of the bucket blowing down towards and across the mirror as with my set up vibration will be a huge issue being an all steel truss telescope sitting on top of a "shakey" sun deck.
His previous email is below which also offers some other alternatives:
-------------
Dear John,
A 2" thick mirror (of any diameter) will certainly benefit from forced
airflow, and the most noticeable improvement will be for planetary
observing. Whatever fan configuration you come up with, it must be able to
run continuously during the observing session without inducing
microvibration. Remember, the main benefit of blowing air over the mirror is
that it keeps the mirror's temperature much closer to the ambient
temperature, and of course the ambient temperature is continuously falling.
This requirement will guide your design, and immediately eliminate some
possibilities (e.g., mounting fans directly to the mirror bucket). Keeping
this "delta T" low is what eliminates (or weakens) the boundary layer.
For your scope, the solution that would require the least hacking and
cutting would be to simply clip on two or three small fans up at the top rim
of the mirror bucket, and roughly aim them down into the bucket. The airflow
recommendations I worked out would not be applicable, as those are tested
for the more common situation of having the fans located near the mirror's
surface. However, it would not take much airflow to do the trick. Try 30-50
cfm as a start. (A few years ago when I was measuring outdoor mirror cooling
rates, I discovered that even a natural breeze would cool about as
effectively as fans if the wind was in the right direction. So, it doesn't
take much airflow at all, nor does it need to be directed particularly
carefully.)
A more involved, but cleaner looking, fix would be to install three small
(~25-30 mm diameter) fans into the bucket wall. These would be positioned
right above the face of the mirror. Of course, this would involve hole
cutting. You also shouldn't try to mount the fans directly to the bucket
wall, or it will certainly induce microvibration (see my May 2004 S&T
article for how to test for microvibration). Foam gaskets are also only
partially effective. The best solution I've come up with so far is to mount
the fans to a soft elastic sheet material, which is in turn mounted within a
plastic frame. The elastic will absorb and dampen vibration before it can
get transmitted to the bucket.
If you really want to get clever, you can try something like the photo I
have attached. I have not done this solution myself, but I know two people
who successfully implemented this "suspended fan" in their large Dobs. The
suspension wires are aligned with the spider vanes, and two of the wires
also conduct the electrical current. Of course, the fan housing cannot be
larger than the secondary mirror, but this usually isn't a problem since the
fan can be located in an almost perfect location.
It's hard to tell from your photos, but it doesn't look like you have room
behind the mirror for fans.
Finally, if you are really serious about this, consider attaching a thermal
probe to the back of the mirror (temporarily). This will let you quantify
the thermal cooling rates, and let you conclusively determine when/if you've
made an improvement. It isn't as hard to do as you might think (see
http://www.fpi-protostar.com/bgreer/miscpages/probe.htm). The little
handheld device I use costs about $100 US, but even a decent $20
indoor/outdoor electronic thermometer might be good enough. It's not the
absolute accuracy of the device you care about as much as its night-to-night
repeatability.
Good luck with your project. The improvements will be worth your effort,
especially for planetary/lunar viewing.
Sincerely,
Bryan Greer
